Vertebrate Bias

Even the spineless feel pain.

Can invertebrates feel pain? “No” is the scientific consensus thus far on all but octopuses—but that may just reflect an ingrained human bias against “simple” animals. Last spring, Robert W. Elwood of Queen’s University Belfast and graduate student Mirjam Appel caused ripples when they reported that [hermit crabs —those little crustaceans that live in salvaged seashells—appear to experience pain. The two biologists subjected each crab to a slight electric shock delivered by wire through a hole in its shell. The shockee hastily exited its shell and rubbed its abdomen where it had been zapped—much as we and other vertebrates respond to painful stimuli.

Now Elwood and Appel have gone further, showing that hermit crabs not only seem to feel pain, but can remember it, too. The team’s shocked subjects usually reenter their mobile homes, but during the twenty-four hours following the bad experience they are more likely than unshocked crabs to inspect an empty shell nearby. In fact, a half hour after the shock, they’re also more likely to abandon their old shell altogether and trade it in for the new one.

Scientists usually invoke reflex, as opposed to pain sensation, in explaining invertebrates’ responses to noxious stimuli. One key criterion they use to identify pain objectively in vertebrates is the creation of memories that affect such decisions as the hermits’ shell swap. By that measure, Elwood and Appel argue, hermit crabs—and perhaps other crustaceans—probably do feel pain. (Animal Behaviour)